Sulfate transport in Neurospora crassa: regulation, turnover and cellular localization of the CYS-14 protein

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Abstract
Uptake of inorganic sulfate in Neurospora crassa is governed by the sulfur regulatory circuit and is under the control of positively and negatively acting regulatory genes. Two genetically and biochemically distinct systems are responsible for the uptake of sulfate from the environment. One of these, sulfate permease II, encoded by the cys-14 gene, functions primarily in mycelia. A defined region of the CYS-14 protein was highly expressed in Escherichia coli and purified. Anti-CYS-14 antibody was produced and used to detect the CYS-14 protein in N. crassa extracts. The CYS-14 protein has an approximate molecular weight of 95K, in agreement with its calculated size based on its predicted amino acid sequence. The steady-state level of the CYS-14 protein is highly regulated in wild-type mycelia and constitutive in an scon-1 mutant, whereas no CYS-14 protein could be detected in a cys-3 mutant. Following the accumulation of the cys-14 mRNA, that reaches its maximum in about 6 h, the CYS-14 protein accumulates to a maximum level in about 8 h after derepression. During conditions of sulfur repression, the CYS-14 protein turns over with a half-life of approximately 2 h. The CYS-14 protein appears to be localized in the plasma membrane, suggesting that it functions as a sulfate ion transporter.

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